CANDU Reactors

CANada
Deuterium Uranium (CANDU) reactors use heavy water as a coolant instead of regular old light water.
These were invented and developed in Canada in the 1960’s, when Canada decided that they did not
want to build enrichment plants or large pressure vessel manufacturing capabilities. Heavy water can
be made from regular water, but it takes a lot of energy to do so. But, heavy water absorbs neutrons
much less probably than regular water, so it’s significantly easier to sustain a neutron chain
reaction with heavy water. This allows CANDU reactors to operate with natural uranium (no enrichment necessary). As of 2010, there are 29
operating CANDU reactors, with 17 of these in Canada.

Pros

No enrichment required. Heavy water allows natural uranium to be burned directly

Bypassing enrichment, no depleted uranium tails are made. This allows CANDUs to get very efficient
use of the uranium resource, higher than all other water-cooled reactors

CANDUs can be refuelled without shutting down the reactor, avoiding the downtime that most other reactors require

Cons

Heavy water is expensive

When regular water absorbs one neutron, it becomes heavy water. But when heavy water absorbs one
neutron, it becomes tritium (H-3), which is a low-level radioactive hazard. Tritium is difficult to
contain and enters biological systems readily. CANDUs make more tritium than light-water reactors
because they have so much heavy water.

In this figure, we plot the probability that hydrogen and deuterium will absorb a neutron (hurting
the chain reaction). As you can see, H-2 (deuterium, the constituent of heavy water) eats up
neutrons with 3 orders of magnitude less probability.